The invention relates to an unreliability detector apparatus and to a reproduction apparatus provided with the unreliability detector apparatus. Unreliability detector apparatuses are generally provided with two comparators in which the input signal is compared with a positive and a negative threshold level respectively that lie between the maximum positive signal level and the zero signal level and between the maximum negative signal level and zero respectively.
In a bit detector, it is commonly decided whether the signal is positive or negative. A positive signal is said to correspond with a +1 and a negative signal with a -1. Ideally, signal values at the decision moments are +100% or -100%: that is, without noise and other disturbances. Decisions, which are taken from the signal around the +100% or -100% values are almost certainly correct. Decisions, which are taken from the signal near the zero level are uncertain.
With two threshold levels, equal to +V.sub.d and -Vd respectively, a decision &gt;+V.sub.d is said to correspond with +1 and a decision &lt;-V.sub.d is said to correspond with -1.
Decisions between -V.sub.d and +V.sub.d are less certain. They are called erasures. The number of erasures depends strongly on the levels of the decision thresholds. For increasing threshold values, the erasure rate will increase very strongly.
If we consider erasures as uncertain bit decisions, we can try to make these uncertain bit decisions more certain. This is what is done in a dual decision feedback equalizer (DDFE). DDFEs are well known in the art. Reference is made in this respect to international patent application no. IB97/00792, which corresponds to copending U.S. application Ser. No. 08/886,997 filed Jul. 2, 1997.